Tuesday, 28 July 2015

I'm very mindful of speaking clearly, when I think people might be listening. It's extremely important to communicate ideas precisely, so that they can be understood with minimal effort on the part of your audience.

In my parallel PHP podcast and talk, and a previous blog post, I gave clear explanations of asynchronous and parallel concurrency.

I'm going to assume you haven't seen any of that, and repeat the explanation, with pretty pictures and some more thoughts ...

Let's imagine we have three distinct tasks to execute, the nature of those tasks is unimportant for the explanation.

Synchronous Execution

Following is a diagram of the synchronous execution of those tasks:

Fig 2: Synchronous Execution

This is the model we are all used too; instructions are executed in a linear fashion, one after the other, task after task.

This is easiest for everyone; It's easiest for the programmer using a language, and easiest for the engineers of the language.

Asynchronous Execution

Following is a diagram of the asynchronous execution of those tasks:

Fig 3: Asynchronous Execution

We can see that the instructions for each task are interleaved with each other, because of this, the tasks can be said to run concurrently with respect to each other.

This is what asynchronous concurrency looks like.

On the face of it, it takes the same amount of time to execute asynchronous code as it does to execute synchronous code.

Asynchronous concurrency has it's most appropriate use case in I/O bound code: Synchronous code that is I/O bound spends a considerable time waiting for hardware to become available. Using non-blocking API's, and interleaving instructions, means that you can eliminate that waiting: When synchronous blocking code would have waited, asynchronous non-blocking code is ready to execute the instructions for another task. This reduces the overall time it takes to execute your tasks.

This is considerably more messy than synchronous code, from both the perspective of programmer and language engineer; Whether it's worth chopping and dicing your instructions such that they can be interleaved is a question you should always ask.

Parallel Execution

Following is a diagram of the parallel execution of those tasks:

Fig 4: Parallel Execution

We can see that the tasks more closely resemble their synchronous counter parts, but run concurrently with respect to time.

The only way to do this is by utilizing more than one thread of execution.

This is what parallel concurrency looks like.

In theory, our code is three times faster than it is when executed synchronously. In practice, given the ability to execute in parallel we invariably want our tasks to communicate with each other. This introduces overhead associated with synchronization (locking), not only costing time, but considerable cognitive overhead for any real world parallel code.

Giving a best use-case is much harder than with asynchronous concurrency, because the applicable domain of parallel concurrency is vast in comparison.

What you can say is that you should aim for that diagram as an ideal, you should design your tasks such that they are, as far as possible, isolated and so incur the minimum possible overhead from synchronization.

Applying Knowledge

Imagine, if you will, a scientist who says that since the universe has awareness within it, the universe itself is aware.

This is a pretty blunt example, I'm sure everyone can see the error in that logic.

But what if the assertion that the universe is aware was accompanied by "a kind of word salad" which somehow lends superficial plausibility to the idea.

I find it hard to imagine there is anyone who cannot understand the key differences between these models. I'm sure it's crystal clear in the readers mind, the differences and properties of these models are easily understood.

What might not be so easily understood, is how to apply that knowledge to a bigger system, with more complex ideas.

How do we describe the execution of a task which was offloaded to Gearman, or some other process; Can't we describe that as asynchronous concurrency (since the operating system might interleave the tasks) ?

That's confusing the universe (system) with what is going on in the universe (system).

No matter the size nor complexity of the system, the definitions given above stand.

If you are interleaving instructions you are achieving asynchronous concurrency, if you are executing instructions in parallel, you are achieving parallel concurrency.

It has to be that simple; When we skew the lines, we describe something parallel as asynchronous, or vice versa, it tastes like word salad, and makes it difficult to understand what is really going on.

A long time ago, I used the words parallel and asynchronous interchangeably in some example code I have distributed, and I could whip myself for doing so. I done so at a time when I just wasn't aware of the confusion surrounding these terms, and assumed, wrongly, that I would be understood.

If you have a blog, or a podcast, or someone you are educating, or any reason to talk about this stuff, I implore you to triple check your language and ideas, before communicating them, and I'll be doing the same.

Wednesday, 22 July 2015

In real life, I'm a pretty awkward guy at first; I'm prone to stuttering, have no sense of direction, I forget names, the faces of people I've met, places I've been, and find it impossible to be assertive, or confident in any way.

I'm pretty much a mess.

Some of these symptoms disappear once I get to know you better, but first impressions count for a lot. My small circle of friends just accept, thankfully, that they have to help me navigate the world if they want me around, and sometimes have to wait for the end of a sentence.

I'm okay with it, I don't live in the real world, so don't have to deal with real life very much. The written word, including code, is not usually awkward for me, it flows freely, though it may not always make sense.

I know I'm not alone, I know lots of people who work in Tech are the same; They will confidently talk about the pros and cons of their favourite systems in IRC all day long, but put them in a real life situation and they fall apart, can't make decisions and don't know who or where they are.

While I am at peace with my acute awkwardness, it so happens that I hugely admire anyone who is able to get up on a stage and hold an audience. It's worth noting that if it weren't for this breed of individual, there are things I would never have understood properly.

Not so many months ago, in a burst of uncharacteristic confidence, I decided I would submit a talk to the inaugural PHP South Coast. This is as local as a conference can get for me, there would be friendly faces (I might not know their names, or recognize them at first), and I chose a subject matter I know a fair amount about, parallel PHP.

The plan was to practice at my local meetup in Hampshire, however, because of real life that never happened.

In the last couple of weeks leading up to the conference my beloved partner and I discussed how the talk should go.

I arranged to drive two friends to the conference, and we thought we could go through my talk in the car. However, it was 7am and we wanted bacon, so that didn't happen either.

So there I was, scheduled to speak after the keynote from Cal Evans, having not practised talking to anyone, not even one person. A few of us got around a table, and I quickly ran through the outline of the talk, asked if anything was missing, and we discussed, for about 180 seconds, questions that could be asked.

I lapped up the keynote, it was funny, even emotional, well produced ... I totally forgot I was up next.

I went straight into the room where I was speaking, then left, walked around to the other side of the room, entered again, then left, went to the other side of the room and finally got my shit together and approached someone that looked like staff.

We couldn't get my laptop to work, which was embarrassing. A working laptop was hooked up, I got the attention of the room and heard myself shakily enter the world of the speaker.

I eventually got control of my voice, managed to get a couple of laughs, and I think confidently answer questions at the end.

It was over, I literally ran outside to a corner and smoked by myself, caught my breath, and began to look forward to the rest of the conference.

A short while later I tweeted:

From the comfort of my office chair, the awkwardness of the experience for me is overshadowed by the sense of pride that I done it.

It may not have been the best talk; I don't know if I stuttered during, or missed anything out. I know that it was short and the audience was left wanting more, and I was left looking forward to having the opportunity to speak again, and improving.